The invention relates to a method for the testing of an exhaust gas recirculation system of an internal combustion engine.
In order to further reduce the fuel consumption of spark-ignition internal combustion engines, increasingly frequent use is being made of internal combustion engines, which in addition to operation with stoichiometric mixture can also be operated with lean-burn combustion. In spark-ignition internal combustion engines with lean-burn combustion the excess-air factor is set as high as the load demand on the internal combustion engine will allow; at times of low load demand the fuel-air mixture, on which the internal combustion engine is run, may have lambda values of 3 and more in stratified charge operation.
In such internal combustion engines special measures are necessary in order to meet required exhaust emission limits, since otherwise the quantities of NOx emitted would be too large. This also applies to diesel internal combustion engines. In addition to the use of NOx storage catalytic converters, which owing to their coating are capable in a storage phase of absorbing NOx compounds from the exhaust gas produced in lean-bum combustion, and with the addition of a reducing agent in a regeneration phase of converting these into harmless compounds, so-called exhaust gas recirculation systems are also known. In such exhaust gas recirculation systems a proportion of the exhaust gas flow is mixed with the fresh charge flowing into the cylinders. Since exhaust gas is an inert gas for the combustion process, this reduces the untreated NOx emission of the internal combustion engine. The recirculated exhaust gas flow, the so-called exhaust gas recirculation rate, is generally controlled by means of an exhaust gas recirculation valve connected into the return line.
Such an exhaust gas recirculation system is an emission-related component. Under current and pending regulations, such components are to be subjected to testing with the internal combustion engine running, since a failure or defective operation of the exhaust gas recirculation system might lead to drastic deterioration of exhaust emission characteristics of an internal combustion engine and to exceeding of prescribed limits.
One component of an exhaust gas recirculation system particularly at risk of failure and leading in particular to increases in exhaust emissions in the event of failure is the exhaust gas recirculation valve, which serves to adjust the exhaust gas recirculation rate. A pressure sensor, which is arranged in the inlet pipe and registers the inlet pipe pressure, has hitherto been used for diagnosis of the exhaust gas recirculation valve. At the same time the air intake mass is determined by an air-flow sensor. From the air intake mass it is possible to calculate the inlet pipe pressure downstream of a throttle valve of an internal combustion engine to be expected for a certain position of the exhaust gas recirculation valve. Should a difference occur between the measured and the calculated inlet pipe pressure, a defective exhaust gas recirculation valve is diagnosed. This principle is described, for example, in DE 44 06 281 A1.
Detection of the working of an exhaust gas recirculation system from the smooth running of the internal combustion engine, on the principle that the frequency of misfiring or rough running of an internal combustion engine increases with the exhaust gas recirculation rate, is disclosed by DE 42 16 044 A1.
DE 36 24 441 A1 furthermore discloses a method for adjusting the exhaust gas recirculation rate with an internal combustion engine idling and monitoring the fluctuation of the speed of the internal combustion engine. A similar method is also disclosed in EP 0 635 629 A1.
The object of the invention is to specify a method for the testing of an exhaust gas recirculation system in which no pressure measurement in the inlet pipe is required.
This object is achieved by the invention characterized in claim 1.
The invention makes use of the finding that variations in the exhaust gas recirculation rate may have a marked influence on the NOx emissions of an internal combustion engine. If the exhaust gas recirculation rate is now adjusted by a certain amount, it is possible to detect a defective exhaust gas recirculation system from the absence of the change in the NOx emission of the internal combustion engine actually to expected. This concept is suitable for all internal combustion engines fitted with exhaust gas recirculation systems.
This testing is particularly easy to carry out where the NOx emission of the internal combustion engine would otherwise be constant, which is particularly the case in static operating conditions of the internal combustion engine, that is particularly where the temporary adjustment of the load and/or speed of the internal combustion engine remains below a certain, suitable limit.
There are various conceivable approaches to detecting the absence of an NOx concentration to be expected in the exhaust gas from an internal combustion engine. On the one hand it is possible to form a differential concentration from the NOx concentration measured in the exhaust gas prior to and after adjustment of the exhaust gas recirculation rate. This differential concentration obviously depends on the adjustment made to the exhaust gas recirculation rate. If the differential concentration is not forthcoming despite adjustment of the exhaust gas recirculation rate, the exhaust gas recirculation valve is defective. In this case the NOx concentration can be measured at any point in the exhaust line, especially upstream of a catalytic converter.
In order to determine the differential concentration one of these NOx concentration measurements can also be replaced by a modelling of the untreated NOx emissions, it being possible to use known models for this purpose, which from operating parameters of the internal combustion engine estimate the NOx concentration emitted for this operating condition. With such a model value for the NOx concentration it is possible, together with the measurement of the NOx concentration after adjustment, to form the differential concentration, and to use for this purpose either the model value for the NOx concentration prior to adjustment of the exhaust gas recirculation rate or the model value for the operating condition after adjustment of the exhaust gas recirculation rate. In so doing, however, it is advisable that the operating conditions of the internal combustion engine otherwise remain largely constant, since this minimizes the error in modelling of the NOx concentration.
If the internal combustion engine has an NOx storage catalytic converter, an NOx concentration sensor, as is usually provided for controlling an NOx storage catalyst of this catalytic converter, can also be used for diagnosis. The same applies to internal combustion engines with a three-way catalytic converter in the exhaust line. A known arrangement, for example, is a sensor situated downstream of the catalytic converter. Since such an NOx storage catalytic converter generally absorbs the NOx compounds in the exhaust gas, however, it must be ensured in this arrangement for carrying out testing that this absorption temporarily does not take place. This can be achieved in a preferred embodiment of the invention by saturating the catalytic converter to its maximum storage capacity prior to testing. Attainment of the saturated condition can be detected by the NOx concentration sensor arranged downstream, for example through comparison of a modelled NOx concentration with a measured NOx concentration or through suitable interpretation of the gradient of the NOx concentration downstream of the NOx storage catalytic converter occurring during a storage process.
When the NOx catalytic converter is saturated, changes in the NOx concentration upstream of the catalytic converter show up at the converter outlet, so that testing is then possible.
Saturation can be attained very rapidly, particularly if a high untreated NOx emission is ensured upstream of the NOx storage catalytic converter, for example by setting the exhaust gas recirculation rate below a specific threshold or even more preferably close to zero.
Basically, an especially good diagnosis is obtained if the exhaust gas recirculation rate is adjusted from a maximum value to a minimum value. In order to achieve this in the variant with the accelerated saturation of an NOx storage catalytic converter it is necessary, when it is established that the NOx storage catalytic converter has reached saturation, to first increase the exhaust gas recirculation rate from the value below the minimum value, so that it can then be reduced again in order to form the differential concentration.
The timing of the adjustment made to the exhaust gas recirculation rate for testing purposes is in principle not significant. If a progressive adjustment of the exhaust gas recirculation rate is set, the diagnosis has particularly slight effects on the operation of the internal combustion engine, since the change in the smooth running of the internal combustion engine inevitably accompanying the adjustment of the exhaust gas recirculation rate occurs slowly. For the most reliable diagnosis possible, it is best to increase the exhaust gas recirculation rate sharply. This method further has the advantage that the testing takes up only a very limited period of time, so that only a very slight increase in the NOx mass emitted occurs due to the testing.
Advantageous developments of the invention form the subject of the subordinate claims.